Conventionally, a freight container is considered an article of transport equipment having an internal volume of 1 m.sup.3 (35.3 ft.sup.3) or more. A freight container is intended for repeated use, and it is specifically designed to facilitate the carriage of goods by one or more modes of transportation, without intermediate reloading. A freight container may be fitted with devices permitting its ready handling, such as its transfer from one mode of transport to another. (In the context of the present application, the term "freight container" includes neither vehicles nor conventional packaging.)
An ISO container is a freight container complying with relevant ISO container standards in existence at the time of its manufacture. The ISO is an international standards setting organization, and compliance with its standards is not mandatory. International Standards ISO 668 (5th edition) and ISO 1496-3 (4th edition) are hereby incorporated by reference.
The present application particularly concerns freight containers used to transport pressurized materials such as, for example, pressure liquefied gases including chlorine, anhydrous ammonia, and fluorocarbons. Fluids such as these are shipped in tank containers with a maximum allowable working pressure between 100 and 500 psi. (The upper limit, 500 psi, is not a theoretical limit, but a regulatory one, and the applicant expects that if and when the pertinent regulations allow higher pressures, freight containers will be built to sustain such higher working pressures.)
Freight containers, including the freight container of the present invention, for the transport of pressurized materials such as pressure liquified gasses are intended to be mounted on a transport vehicle (such as a truck, boat, or railroad car), before or after being filled with a pressurized material, and then transported to a remote location. In most countries, freight containers must be approved for use by a competent authority (or its designated body) appointed by the specific country's government. For example, in the United States, these freight containers must be approved by the Department of Transportation (D.O.T.). Further in most countries the competent authority adopts in whole or in part, a recognized pressure vessel code. For example, the U.S. D.O.T. has adopted the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, with some additional limitations.
A freight container for a pressurized fluid, i.e., a tank container, includes a tank and a framework surrounding the tank. The tank includes various pipes and fittings which are designed to contain the cargo carried and to permit the tank to be filled and emptied. The tank may be formed from a cylindrical shell and two heads, one closing each end of the cylindrical shell. The dimensions of the shell include an outer radius R.sub.o and an inner radius R.sub.i, the difference therebetween defining the shell's thickness T.sub.s.
The shell and heads of a tank container are made of a material meeting the requirements of the approved pressure vessel code or approved by the competent authority. Typically in the United States the shell and heads of tank containers have been made from a high strength steel, SA612N, having an ultimate tensile strength (S.sub.u) of at least 81,000 psi.
The framework of an ISO freight container for pressurized fluids includes tank mountings, end structures and other load-bearing elements which are not present for the purposes of containing the fluid. The framework functions to transmit static and dynamic forces arising out of the lifting, handling, securement, and transporting of the freight container as a whole. The framework includes eight corner fittings (four top corner fittings and four bottom corner fittings), rails, posts, and braces which form its base structure, its end structure and its side structure and satisfy the requirements of ISO 1496-3 Sections 5.1-5.5. In the context of the present application, the term "ISO frame" means a framework which satisfies the framework requirements of these sections.
An ISO freight container for pressurized fluid may also include certain additional components depending on the intended use of the container. For example, if the pressurized fluid is temperature sensitive and/or if the transportation will occur in a temperature extreme environment (i.e., hot or cold ambient temperatures), the freight container may include sunscreens, linings, jacketing (cladding), insulations, air baffles, etc.
In the past, the tanks of such freight containers for fluid under pressure have been designed and constructed in accordance with a recognized pressure vessel code, which in the United States is Section VIII, Division 1, of the ASME Boiler and Pressure Vessel Code covering unfired pressure vessels. The entire disclosure of this Division is hereby incorporated by reference. When these tanks are used at normal environmental conditions of temperature and pressure to hold and transport fluids, the minimum thickness T.sub.s of the shell has been determined by the following equation: EQU T.sub.s .gtoreq.(P R)/(E S.sub.DIV. 1 -0.6 P)
where
P=the internal design pressure for the tank;
R=inside radius of tank's shell;
S.sub.DIV. 1 =maximum allowable stress=S.sub.u /4;
S.sub.u =ultimate tensile strength; and
E=joint efficiency.
The joint efficiency, E, has a value of between 0 and 1, depending on the extent of radiography of the welded joints. When all welded joints are fully X-rayed, E has a value of 1 and essentially drops out of the equation. (In Division 2, all welded joints are required to be fully X-rayed, so this factor does not appear in the equation, which is given below.)
These prior art freight containers have satisfied the competent authorities in various countries concerned with approval of freight containers, including the United States Department of Transportation which is commonly viewed in the industry as having the most stringent approval requirements. Again it is noteworthy that the ASME Boiler and Pressure Vessel Code is not a permanent, standard and is subject to change from time to time. It is anticipated that the maximum allowable stress for Division 1 will be increased from its present value of S.sub.u /4 to S.sub.u /3.5. This would allow the shell to be proportionately thinner, and freight containers will be built to this specification when the change becomes effective.
Tank containers made according to Division 1 of the ASME Boiler and Pressure Vessel Code, Section VIII, which have a capacity of about 4500 U.S.W.G. (U.S. water gallons) and a design pressure of between 335 and 400 psi have had a tare weight of between about 17,000 lbs and 20,000 lbs. This means that when filled to capacity and placed on a truck for transport over a highway, the tank container can easily cause the truck to exceed the weight limits established for such roads. Perhaps the most restrictive country in this regard is Japan, where a tank container should not exceed 53,000 lbs. when loaded. As a result of such load limits, many tank containers can be filled only partially, depending on the density of the fluid being shipped, and this can make them inefficient.
The present invention provides a novel ISO freight container having a tank design which results in a decrease in the freight container's tare weight. In a preferred embodiment, the present invention provides a freight container for transporting a fluid at a pressure P, typically between 100 psi and 500 psi. The freight container includes a tank and an ISO frame. The tank is made with a shell and heads that have an ultimate tensile strength (S.sub.u) of 81,000 psi. The shell of the cylindrical vessel has a thickness T.sub.s given by: EQU T.sub.s .ltoreq.(P R)/(E S.sub.DIV. 1 -0.6 P)
where
P=the internal design pressure for the tank;
R=inside radius of tank's shell;
S.sub.DIV. 1 =maximum allowable stress=S.sub.u /4;
S.sub.u =ultimate tensile strength; and
E=joint efficiency;
and substantially equal to EQU T.sub.s =(P R)/(S.sub.DIV. 2 -0.5 P)
where
P=the internal design pressure for the tank;
R=inside radius of tank's shell;
S.sub.DIV. 2 =design stress intensity=S.sub.u /3
S.sub.u =ultimate tensile strength.
The shell is manufactured to the above thickness with a typical manufacturing tolerance of .+-.6%.
Freight containers according to the present invention have satisfied the requirements of The United States Department of Transportation. Thus, a freight container according to the present invention may be mounted on a transport vehicle (such as a truck or railroad car), before or after being filled with a pressurized fluid, and then transported to a remote location. Freight containers according to the present invention have a tare weight approximately 2000 lbs less than comparable prior art freight containers where both have a capacity of about 4500 U.S.W.G. and a design pressure of 335 to 365 psi.
The present invention provides these and other features hereinafter fully described and particularly pointed out in the claims, the following description and annexed drawings setting forth in detail an illustrative embodiment of the invention, this being indicative, however, of but one of the various ways in which the principles of the invention may be employed.